The present invention relates to methods and apparatus for treating photoresist materials using electrodeless UV lamps. The present invention finds particular application in the preparation of circuit boards.
Processes for forming resist patterns on the surfaces of substrates typically comprise forming a photo-sensitive layer on the surface of the substrate, irradiating portions of the photo-sensitive layer with actinic light, and developing the irradiated layer. If the solubilization of the photoresist increases when exposed to actinic light, it is referred to as a xe2x80x9cpositive-actingxe2x80x9d photoresist; the relatively high molecular weight positive-acting photoresist material depolymerizes, or undergoes breakage of the polymer bonds, upon exposure to actinic radiation thereby rendering the treated compound easily dissolved by developing solution. If the solubilization of the photoresist decreases when exposed to actinic radiation, it is referred to as a xe2x80x9cnegative-actingxe2x80x9d photoresist; the relatively low molecular weight negative-acting material crosslinks upon exposure to actinic radiation and, thus, it is the non-treated compound that dissolves upon exposure to the developing solution.
Photoresists are often used to protect the underlying substrate from the effects of a subsequent etching process. Defects in the resist pattern, such as inadequate coverage over certain parts of the substrate or inadequate development of the irradiated layer, can result in problems in the completed product. Accordingly, it is important to employ a photoresist whose irradiated layer can adequately be developed and that forms a uniform layer over all surfaces of the substrate to be protected.
Photoreactive polymers are particularly useful as binder resins in photoresist compositions employed in photodevelopment of electronic components such as circuit boards and other products. Positive-acting resists are often preferred in the manufacture of circuit boards; defects that will occur in the manufacture of circuit boards are easier to repair when positive-acting resists are used as compared to when negative-active resists are used. The irradiation of the photoresist, in the case of circuit board manufacture, often occurs through a glass or plastic cover sheet. The use of xe2x80x9cmasksxe2x80x9d or xe2x80x9cart workxe2x80x9d prevents exposure of desired areas of the photoresist to UV light.
The present invention is directed to the use of electrodeless UV lamps to effect treating of photoresist compositions. Both positive and negative photoresists can be treated according to the present invention. A positive resist, as discussed above, is a relatively high molecular weight composition, such as a polymer, that depolymerizes upon exposure to UV light. The polymer bonds break thereby reducing the overall molecular weight of the composition. The positive photoresist composition that is exposed to UV light is therefore more easily dissolved by a developing solution. In this manner, the portion of the composition that has been exposed to UV light is removed. Negative photoresists, in contrast, are relatively low molecular weight compositions that crosslink upon exposure to UV light to form high molecular weight polymers. The material that is not crosslinked, i.e. that which is not exposed to UV light, is removed during the developing step.
The present methods find particular application in the preparation of electronic circuit boards. To this end, the present invention is further directed to apparatus and methods for making a circuit board. While the present invention is directed to the treatment of both positive and negative photoresists, positive-acting photoresists are often preferred in the manufacture of circuit boards. The surface characteristics of positive photoresists are often superior to negative photoresists, with fewer mechanical defects. In addition, positive-acting photoresists often handle better than their negative counterparts. The energy needed to break bonds in the case of positive photoresists is typically much higher than the energy needed to form bonds, or promote crosslinking in the case of negative photoresists; this increased energy requirement typically necessitates a longer exposure time to the light source. This can result in a much longer manufacturing time per piece, particularly when using automated systems. Thus, notwithstanding the benefits achieved with a positive photoresist, because of the extended exposure time mass production of circuit boards and other products often favors the use of negative photoresists. The present invention addresses this issue. Because electrodeless UV lamps operate at a higher intensity, their use may significantly reduce the amount of UV exposure time for treating positive photoresists.